Hyperfiltration is a process whereby pressure is applied to one side of a semi-permeable membrane, causing a solvent (commonly water) to pass through the membrane while a solute (often a salt) is retained. To overcome the natural drive for solvent to move from low concentration to high concentration, the applied pressure must exceed the osmotic pressure. For this reason, the term “hyperfiltration” is often used interchangeably with “reverse osmosis.” For purposes of this specification, hyperfiltration encompasses both reverse osmosis (RO) and nanofiltration (NF) processes.
Hyperfiltration membranes are most commonly used in a spiral wound configuration, as this configuration allows a large amount of membrane area to be packed into a small volume. A typical spiral wound module (2) is illustrated in FIG. 1. One or more membrane envelopes (4) and feed spacer sheets (6) are wrapped about a central permeate collection tube (8). The envelopes (4) comprise two generally rectangular membrane sheets (10) surrounding a permeate carrier sheet (12). It is usual that this “sandwich” structure is held together by glue lines (14) along three edges (16, 18, 20) while the fourth edge (22) of the envelope (4) abuts the permeate collection tube (8) so that the permeate carrier sheet (12) is in fluid contact with small holes (24) passing through the permeate collection tube (8). Construction of spiral wound modules is described further in U.S. Pat. Nos. 5,538,642, 5,681,467, and 6,632,356, which are incorporated by reference.
Large arrows in FIG. 1 represent the approximate flow directions (26, 28) in operation for feed and permeate. The direction of feed flow (26) is from the inlet end (30) to the outlet (reject) end (32) across the front surface of the membrane (34). The permeate flow direction (28) is approximately perpendicular to the feed flow direction (26). The actual flow paths and velocities vary with details of construction and operating conditions. Under typical operating conditions, a module might demonstrate feed velocities of 0.15 m/sec and permeate velocities near the tube of 0.04 m/sec. Feed velocities decrease from inlet end (30) to outlet end (32) because some feed liquid is lost to the permeate side. For a well constructed module, permeate velocities similarly increase from the back glue line, where they approach zero, to a maximum velocity at the permeate tube.
Spiral wound modules are generally placed inside of a cylindrical pressure vessel for operation, as illustrated in FIG. 2. It is common that up to eight spiral wound modules (2) may be combined in series within a pressure vessel (40). Pressure vessels (40) have ports (42,43) on both ends for passing feed axially through each of the modules (2) in series and at least one additional port (44) for removing permeate solutions. Permeate collection tubes (8) from adjacent modules (2) are joined by interconnectors (46) having at least one permeate seal (48), and the effect is to approximate one long module within a vessel (40). For the purposes of this specification, a vessel's permeate collection region (50) includes the volume surrounded by permeate collection tubes (8) in series, their interconnectors (46), and their vessel end adapters (52). (Vessel adapters (52) typically join a permeate collection tube (8) to a vessel end cap (54) to allow permeate to exit the vessel.) A pressure vessel can be further combined in series or parallel with other pressure vessels to create a membrane filtration system.
It is typical that manufacturers of spiral wound hyperfiltration modules test modules individually and specify a salt rejection after 20-30 minutes. While small changes in performance may actually continue for days or months, the 20-30 minutes allows measurements to be made under conditions that approximate steady-state. For brackish water modules, a common test uses 2000 ppm NaCl and an applied pressure of 225 psi. Seawater products are typically tested with 32000 ppm NaCl and an applied pressure of 800 psi. FilmTec's NF270 module is tested at 70 psi with 2000 ppm MgSO4. An “intact” module, without membrane or construction defects, typically demonstrates between 0.3% and 3% maximum salt passage in these standard tests. Since hyperfiltration allows some passage of salt through even intact membranes, these tests are not especially sensitive to the macroscopic defects that may result from module construction problems. Further, even when high salt passage is observed, these standard tests provide no information on the type or source of a defect.
As indicated by arrows in FIG. 3, there are several particularly common regions for leaks into the permeate flow path. Regions at the back (60) and sides (62,64) of the permeate carrier sheet (12) correspond to defective glue lines (14), allowing a direct path for feed to enter the permeate. A region (66) near the edge abutting the permeate collection tube (8) corresponds to the membrane fold and has been a common source of leaks, particularly for modules subjected to very rigorous and frequent cleaning cycles. At the inlet and outlet ends of the module, near the permeate tube (8), regions (68,70) corresponding to insert leaks (where a leaf pulls away from the module in construction) may cause high salt passage. The membrane itself may also generally have high salt passage or it may have localized defects such as scratches and pinholes, and these may result in feed liquid passing into the large center region (72) of the permeate channel.
The location of defects within a spiral wound module can be difficult to discern. In some cases, autopsy and dying can reveal the position of defects (“Membrane Element Autopsy Manual,” Water Treatment Technology Program Report #17, U.S. Bureau of Reclamation, 1996). However, autopsy is a destructive and time consuming procedure, and the delay associated with obtaining results means that it rarely results in information that can be used to correct an existing problem in fabrication.
Hyperfiltration modules are most commonly used to remove salts from water. These membranes also remove specific larger impurities of interest (e.g. Giardia, Cryptosporidium, viruses). Thus, hyperfiltration can produce potable water from surface water while limiting the need for disinfectants. These membranes are also used to treat municipal waste waters for direct and indirect potable reuse. However, due to concerns over integrity, hyperfiltration is always one of several steps used to treat these waters, and its actual impact on the removal efficiency for larger particles is generally undetermined.
Complete removal of any species by membranes requires both that all of the product water pass through the barrier layer and that the barrier layer is defect-free. J. Lozier et al. teach that key areas for virus and cyst passage within a hyperfiltration module as imperfections in the membrane sheet, imperfections in the glue or heat seals of the membrane leaf, and imperfections in the membrane at the area of attachment to the product water tube. (J. Lozier, et al., “Microbial Removal and Integrity Monitoring of High-Pressure Membranes”, AWWA Research Foundation, 2003). The standard salt rejection test used by manufactures lacks the sensitivity to detect the defects that are more prone to passing larger particles. Additionally, several primary causes for leaks are external to modules, particularly the interconnectors that join adjacent modules and connect modules to external piping. An effective test method to be used in systems would need to evaluate the module and all components surrounding it. The test would also ideally not interrupt system operations, not only because of resulting decreased productivity, but also because spiral wound modules are most reliable when operated continuously.
In recent years, a number of methods for testing integrity of UF, MF, and RO systems have been proposed and demonstrated, and these have been reviewed in several publications. See, for example, Lozier, et. al., op. cit.; M. M. Nederlof, et. al., “Integrity of membrane elements, vessels and systems,” Desalination, 113 (1997), 179-181; M. W. Chapman, et. al., “Methods for monitoring the integrity of reverse osmosis and nanofiltration membrane systems,” Desalting and Water Purification Research Report 55, Bureau of Reclamation; and S. Adham, et. al., “Monitoring the integrity of reverse osmosis membranes” Desalination 119, (1998), 143-150.
The integrity of modules is often assessed by means of air flow measurements. These air flow tests are usually based on the bubble point method, and variations have been described in several patents. See, for example, U.S. Pat. Nos. 6,202,475, 6,228,271, 6,324,898. Pressure or vacuum is applied to one side of the membrane, causing air to flow freely through large holes. With hyperfiltration modules, a standard test method is to apply vacuum on the permeate side of the membrane and observe air passage as decay in that vacuum over time. (ASTM D6908-03, “Standard Practice for Integrity Testing of Water Filtration Membrane Systems”, ASTM International, West Conshohocken, Pa., (June 2003), 1-13) This may be done with wet or dry modules. However, this standard method is generally limited to detecting holes greater than about 2 microns, and a system must temporarily be taken off-line to be tested. In some cases, air flow tests can provide information about the approximate location of leaks within a system and even within a module. For instance, pressurizing the permeate tube of a wet spiral wound hyperfiltration module may result in bubbles at the scroll end with positions indicative of certain leak types. However, this method is time consuming and difficult to automate, and its sensitivity is greatly limited by the low pressures required to avoid membrane delamination.
Laine et al. teach use of an acoustic sensor to detect cut fibers in UF systems. (Laine, J. M. et al, “Acoustic sensor: a novel technique for low pressure membrane integrity monitoring,” Desalination 119 (1998), 73-77.) An advantage of this test is that a system can continue to produce treated water as while the system is evaluated. Holes with 0.5 mm diameter may be detected, but sensitivity depends strongly on background room noise; further, the process only works during dead-end filtration. In addition, the method of Laine et al. provides some information on approximate leak location using a separate sensor on each module. However, it is not clear that acoustic sensing could be effectively adapted to cross flow spiral wound modules; it would certainly not detect the wide range of hole types and sizes that are present in hyperfiltration modules.
A variety of natural constituents of feed water have been used to continuously monitor for anomalies in membrane systems. Chapman, et. al, teaches indicators of new system leaks include increased levels of particle counts, TOC, turbidity, TDS, divalent anions or cations, colored substances detected at 455 nm, and substances detected at 254 nm (organics, humic and fulvic acids). Particle counting is one of the most common methods employed in MF/UF systems, but this would not be as appropriate for hyperfiltration when feed water has been pre-treated. For each of these methods, sensitivity depends on the constituents in the natural feed water and their consistency over time. Due to changing membrane, changing feed, or changing process conditions, small changes in performance are difficult to perceive.
ASTM standard (D 6908-03) incorporates the teachings of Chapman, et. al. and Lozier et. al., where a well rejected challenge species, that is also easily detected in the permeate, is added to the feed. Chapman challenged RO membranes with Allura Red (FD&C #40). Lozier has mixed in both Rhodamine WT and 0.02 micron fluorescent microspheres into system feeds. As recommended in the ASTM standard, permeate samples were collected after modules had been running on the challenged feed for substantial time to obtain relatively static conditions. System leaks were indicated by an elevated ratio of permeate to feed concentration. Chapman and Lozier both found that dye tests were capable of detecting some large leaks but tests were not sufficiently sensitive to unambiguously detect all defects. For hyperfiltration membranes sensitivity of dye tests is limited by diffusion of dye through the membrane and by issues with disposal of high concentrations of reject solution. Use of fluorescent microspheres is prohibitively costly at present. These challenge tests provide only a single value to indicate failure, and this number gives no information about leak location.
Particularly when other measurements suggest there is reason to suspect that a particular vessel of spiral wound modules has integrity issues, a probing conduit may provide a means to localize a problem to a particular module. A publication (“FILMTEC Membranes: Probing Reverse Osmosis Systems,” DOW Form No. 609-00235-0404, Dow Chemical, Midland, Mich., (Nov., 1997)), describes how a tube may be inserted into the vessel, down a series of connected permeate tubes, so that water may be diverted and analyzed from a particular section of the vessel's permeate collection region. Unfortunately, the process is time-consuming and requires taking the system off line if adapters for the probe are not present. There is a need is for an improved method of detecting leaks in individual spiral wound hyperfiltration modules. This test should be particularly responsive to macroscopic module construction defects that allow passage of viruses and bacteria, but also sufficiently sensitive to detect small holes that might notably increase salt passage. There is also a need for a method that verifies the integrity of a hyperfiltration system in the field, without interrupting water production. For both cases, it is desired that tests be more rapid and have higher sensitivity than existing options. Preferably, tests would indicate the existence of a leak, and also provide information regarding the location and cause of this leak.